Toxoplasma gondii is a widespread parasite of animals that causes zoonotic infection in humans. Although acute infections in healthy adults are normally resolved without serious consequences, infection of immunocompromised individuals can lead to serious complications. Additionally, because chronic infections are not cleared by the immune system or by normal antibiotic treatment, individuals remain chronically infected for life. The presence of semi-dormant tissue cysts that can reactivate predisposes infected individuals to the risk of complications in the event of waning immunity. One of the key features of T. gondii that enables it to persist in the host is the ability to block immune responses, and thereby prevent clearance of the parasite. Among the pathways that are perturbed by T. gondii infection are the signal transducer and activator of transcription (STAT) family of transcription factors. Control of T. gondii infection relies on activation of STAT1 by IFN-?, which is essential to upregulate antimicrobial pathways in both hematopoietic and non-hematopoietic cells. However, when cells are infected by T. gondii before encountering IFN-?, the parasite is able to block STAT1-mediated transcription resulting in reduced control of infection. Global analysis of gene expression reveals that the majority of IFN-?-activated genes are blocked by prior infection with T. gondii. The block in STAT1 signaling occurs in both rodent and human cells, although the molecular basis for this trait has not been previously identified. In preliminary studies we have identified a T. gondii protein that binds to phosphorylated STAT1 dimers and prevents activation of downstream genes. This effector, known as T. gondii Inhibitor of STAT Transcription (TgIST) blocks IFN-?-induced gene expression mediated by STAT1. TgIST is both necessary and sufficient for blocking STAT1- mediated transcription in mouse and human cells. TgIST binds to a host repressor complex that alters chromatin structure. The proposed studies will define the role of host chromatin modifiers in modulating STAT1 transcriptional complexes using gene disruption by CRISPR/Cas9 combined with various reporter assays. Additionally, we will implement a variety of cellular, biochemical, and genetic methods to define the domains necessary for export and trafficking of TgIST to the host cell nucleus, where it is active.